1. Field of the Invention
The present invention generally relates to an apparatus such as a camera in which a photographic film having a magnetic surface is used, and particularly relates to a mechanism for constantly positioning a magnetic head for writing and/or reading magnetic information to or from the magnetic surface of the film, which is apt to move inside the camera body.
2. Description of the Related Arts
Conventionally, there has been provided a photographic film having a magnetic surface and an apparatus, such as a camera, having a magnetic head for writing and/or reading magnetic information with respect to the magnetic surface of the film. Taking a camera for an example, a film set inside the camera is generally free to move in a region between a pair of film guides installed inside the camera body, by which mechanism the film may tilt or move up and down relative to the camera body when the film is wound up on a spool or rewound into a film cartridge. Therefore, if such a film having a magnetic surface is applied to the camera, the signal output may drop down or there might occur signal errors on writing and/or reading the magnetic information relative to the magnetic surface of the film, due to the film moving up and down relative to the magnetic head or due to the film tilting relative thereto which induces some azimuth angles. Consequently, it is necessary to keep the magnetic head in the correct position with respect to the magnetic surface of the film in order to avoid such a malfunction.
On the other hand, it is difficult to keep a positional relationship constant between the magnetic head of the camera and the magnetic surface of the film by adopting a conventional technique of bending the path along which a magnetic audiotape is fed in order to keep the film in precise position with respect to the magnetic head, because the film is far more rigid than the audiotape.
To solve this problem, a mechanism for positioning the magnetic head relative to the magnetic surface of film instead of positioning the magnetic surface of the film relative to the magnetic head, has been presented in Japanese Laid-open Unexamined Patent Publication No. 3-273227.
FIGS. 1 and 2 show the conventional positioning mechanism for positioning the magnetic head relative to the magnetic surface of the film, disclosed therein. FIG. 1 is a rear elevation view of the mechanism while FIG. 2 is a sectional view taken along the line II--II of FIG. 1. In both figures, a reference numeral 11 indicates a camera body, 12 a film pressure plate, 13a and 13b a pair of guide rods, and 14 a magnetic head fixed to the film pressure plate 12. The camera body 11 has an aperture 11d, a pair of rails with surfaces 11a, apart from the front surface of the film pressure plate 12, extending along the upper and lower part of the aperture 11d between a film cartridge chamber and a spool chamber, a pair of first working faces 11b, each of which projects a bit greater amount than each of the faces 11a towards the front surface of the film pressure plate 12 and each of which is located alongside each of the faces 11a, slidably contacting with the front surface of the film pressure plate 12, and a pair of second working faces 11c each of which is located perpendicular to each of the first working faces 11b.
A pair of guide rods 13a, 13b are secured to the camera body 11 by a pair of pad plates 17a, 17b and screws 18a, 18b with the film pressure plate 12 being sandwiched between the guide rods 13a, 13b and the first working faces 11b of the body 11. These guide rods 13a, 13b extend in a crosswise direction of the film 19, with their axial centers being parallel to each other and to the surface of the film 19.
The film pressure plate 12 has a pair of projections 12a, 12b each of which is in the form of a circular arc coaxially located and each of which contacts each of the guide rods 13a, 13b from inside. The film pressure plate 12 is biased upwards by a pair of coil springs 16. One end of each spring is engaged to the film pressure plate 12 and the other end of each spring is engaged to the camera body 11. As shown in FIG. 1, the film pressure plate 12 has a pair of pins 15, secured thereto near its lower edge, with which the lower edge of the film 19 is contacted so that the magnetic head 14 is kept in position with respect to the magnetic surface of the film 19.
Namely, the movement of the film pressure plate 12 is limited by and between the first working faces 11b and the guide rods 13a, 13b in the direction of the optical axis, and the movement thereof is also limited by and between the second working faces 11c in the direction perpendicular to the direction in which the film 19 is fed, thus enabling the pressure plate 12 to rotate about its center of the circular arcs of the projections 12a within a limit.
According to this mechanism, the magnetic head 14 is kept in position with respect to the magnetic surface of the film 19 even if the film 19 tilts or moves up and down relative to the camera body while the film 19 is being fed, because the pins 15 contact with the lower edge of the film 19 by the biasing force of the coil springs 16, which makes it possible that the pins 15 follow the movement of the film 19.
FIG. 3 (I) is a diagrammatic view of the conventional mechanism of FIG. 1, illustrating that the pins 15 contact the lower edge of the film 19. As shown in this figure, one end of the coil spring 16 is engaged to the unillustrated camera body by a pin 16a, 16b secured thereto, respectively, and the other end of the spring 16 is engaged to the film pressure plate 12; therefore, the film pressure plate 12 is supported in a biased manner by the camera body.
If an impulse is momentarily exerted on the camera body in a direction perpendicular to the direction in which the film 19 is fed, as designated by the arrow in FIG. 3, the camera body will be accelerated towards the same direction as that of the impulse exerted thereto at the same time. Supposing that the film 19 is fixed relative to the camera body, the film pressure plate 12 has a tendency to remain stationary at the same location at this moment, because the film pressure plate 12 has its own inertia. In the case where the impulse is relatively large, it causes the film pressure plate 12 and the film 19 to move so that the pins 15 actually move away from the edge facing the pins 15 of the film 19, as shown in FIG. 3 (II), with the result that the impulse exceeds the biasing force of the coil springs 16.
Consequently, with this type of conventional mechanism, it is difficult to keep the magnetic head in precise position with respect to the magnetic surface of the film, when an impulse is exerted on the camera body to move it in a direction.